Well.... The saga takes a turn, perhaps it wasn't the power valve after all. There was no noticeable damage visible on the power valve. I don't a have vacuum kit required to really test it properly. One other fact I have not mentioned up to this point. After the huge backfire I noticed that my idle rpms went up 200, just that 200. I used to idle in gear at around 800. All the sudden I am at 1000. Even after installing the new power valve, the rpms did NOT drop. OK, not sure why but let's back off on the main idle screw and see what happens. I turned it a good turn and half and rpms did NOT drop, nothing. Started looking much closer as to why the rpms would not back off with a good turn on the idle screw. Come to find out I noticed that my choke lever (my "choke" is not hooked up, and I don't have the choke butterfly on) was stuck in the middle position.
Below is what I believe is the zero choke lever position:
Below is what I believe is the full choke lever position:
Below is where I found my choke lever (red line). When in this position and looking at the other side of the carb, there is a "gap" between the main idle screw and the throttle lever hence the throttle does not retract all the way, hence the extra 200 rpms. Sure enough, pull the choke lever to the OFF position and the rpms drop as the throttle lever drops and rests on the main idle screw.
My carb understanding is very basic. I always through the choke was nothing but a rotating flap over the carb, when "on" resulting in less air to the carb hence car runs richer. Obviously there are other things in the formula as things do shift on the other side of the carb when in the choke mode, hence my idle going up. ....choke circuit...???
Could the pressure and rattle of the major backfire cause the choke lever to shift? Bigger question, if that's what it was, why would the car stall on me during the day and not when I took it for a test drive tonight before realizing that choke has shifted? Only thing I can think of is air temp. Today was a nice day, actually I think we hit 70. My car ran at 175 today verses 130 tonight. Could the hotter engine expose the choke-on/rich issue while the cold engine hide it?